Present invention is a means of compensating for the unit-to-unit variation of scanline bow in a multiple ROS color printer system by adjusting the scanline bow of each ROS to a common shape and magnitude. More particularly, for adjusting the scanline bow in an optical scanner to approximately equalize the scanline bows in a multiple ROS printing system.
A raster output scanner (or ROS) conventionally has a reflective multi-faceted polygon mirror that is rotated about its central axis to repeatedly scan a single intensity modulated beam across a photosensitive recording medium in the scan direction while the recording medium is being advanced in the cross scan direction. Typically, a laser generates the light beam and a modulator, such as an acousto-optic modulator, modulates the light beam in accordance with the input information to be reproduced on the recording medium. Alternately,
the laser can produce a modulated beam directly. Typically in a printing system the laser beam rapidly scans the recording medium while the recording medium is moved orthogonally to the direction of the scanning beam to create a raster scanning pattern. The laser beam scan direction is sometimes referred to as the fast scan direction while the cross scan direction, which is the direction of photoreceptor motion, is sometimes called the slow scan direction
Both single beam and multiple beam raster output scanners are particularly useful for high speed printers or multiple color printers. An undesirable character of raster output scanners, however, is scanline bow. For a ROS beam, bow distortions occur from the unavoidable imprecisions in the manufacture and mounting of the lenses and mirrors of the optical elements of the ROS. Scanline bow arises from the very nature of optical scanning systems, where the beam is offset in the cross-scan direction from the ideal horizontal straight line in the scan direction of the scan line on the recording medium. For example an optical aberration such as distortion that varies as the beam scans through different parts of the f-theta lens system can cause scanline bow.
Depending upon the accumulation of optical tolerances, the bow may bend in the middle of the scan line about a central mid-point in either cross-scan direction. A bow where the central mid-point is higher than the rest of the scan line is called a xe2x80x9cfrownxe2x80x9d while a bow where the central mid-point is lower than the rest of the scan line is called a xe2x80x9csmilexe2x80x9d.
As best seen in the force diagram of FIG. 1, the bending device 106 of the mirror mount is located around a center portion of the wobble correction mirror 102 (i.e., the area near the center point CP). Thus, when the setting screw is appropriately adjusted, the bending device 106 applies a force through the two moveable lower abutments to the bottom edge of the mirror 102. These two abutments serve as load points and are symmetrically located about the vertical axis and because of the single set screw apply equal force to the mirror. The top edge of the mirror is restrained by the two fixed upper abutments 116 and 118 along the outer edges of the upper edge. These two abutments serve as fulcrums and are symmetrically located about the vertical axis to apply equal force to the mirror. The moveable load points are much closer to the center of the wobble correction mirror than the stationary fixed fulcrums.
The force applied to the bottom middle edge of the mirror causes the mirror to be bent upward in the vertical axis and results in a local displacment of the optical axis of the wobble mirror in the direction parallel to the optical face and perpendicular to the optical axis of 102 due to this bending of the optical axis, as shown in FIG. 2. That is, the position of the center point CP will move a certain distance vertically out of the plane defined by the horizontal ends of the mirror, to create a xe2x80x9cbowxe2x80x9d or xe2x80x9cbendxe2x80x9d in the mirror 102 without deforming the cross sectional shape of the mirror so that the cylindrical focus of the mirror is not changed during adjustment. Typical mirrors 102 are capable of easily being adjusted in sag by up to 2 mm, depending on the width and the length of the mirror. However, movements of only a fraction of a millimeter are sufficient to correct for scanline bow problems.
Moreover, the magnitude of the scanline bow varies from one optical scanner to another optical scanner. Therefore in a printer with multiple optical scanners, such as a single pass color printer, there can be significant overlay mis-registration of the raster images for the various colors as in FIG. 1 due to differing amounts of bow for the raster image of the different raster scanners.
It is the object of present invention to adjust the bow of each scanner to a common shape so that the raster images from the various scanners in the system will have minimal color overlay mis-registration in the slow scan direction as in FIG. 6.
The prior art has utilized bow adjusters with a different principle from the present invention. In U.S. Pat. No. 5,543,829 by Fisli a mirror is deformed normal to the optical face. This bending of the mirror face causes an incident scanning beam that is reflected from the mirror to be locally translated parallel (relative to the reflected beam from the undeformed mirror). Rumsey et al in U.S. Pat. No. 6,219,082 also bend the mirror normal to the mirror optical face. Another invention that bends a mirror normal to the optical face with mechanical adjusters is U.S. Pat. No. 5,210,653 by Schell and uses a multitude of adjusters to change the surface figure of a mirror.
The present invention utilizes a cylindrical mirror and more particularly the wobble correction mirror to correct the scanline bow of a raster scanning system. It differs from the prior art by 1) adjusting the cylindrical mirror in a direction parallel to the mirror face and perpendicular to the optical axis of the mirror. 2) The mirror must be a cylindrical mirror cannot be a planar mirror as in the Fisli and Rumsey patents. Additionally the present invention closely maintains the cylindrical curvature of the mirror during deformation so that cross scan spot size is not adversely changed during bow adjustment.
FIG. 1 shows a schematic of the wobble correction mirror 102 being deformed parallel to the optical face by forces applied to the bottom edge of the cylindrical mirror at 120 and 122. Two abutments 116 and 118 at the top edge of the cylindrical mirror provide a counter-force so that the mirror will bend in flexure. It can be shown by calculation and by actual measurement that the deformed shape of the mirror along its length is a parabola.
Cross section of the cylindrical mirror in a region between the fixed abutments is schematically illustrated in FIG. 2. The cross section of the mirror before adjustment is illustrated 102 by the solid line and the cross section of the mirror after adjustment by 102. The movement of the mirror cross section is actually displaced parallel to to the mirror face by the present invention and is only shown slightly displaced in the normal direction to more clearly see the outline of the mirror before and after displacement. In the present invention the incident beam 203 is reflected by the undeformed wobble mirror as a reflected beam 205. After the wobble mirror cross element is deformed to position 102, the reflected ray is rotated through a small angle relative to 205 and becomes beam 207. It can be shown by calculation and by experiment that the angular rotation of beam upon displacement of the mirror cross section is proportional to the amount of displacement for small displacements. Moreover the amount of change of the scanline bow is jointly proportional to both the rotation angle and the distance from the wobble mirror and this product is the change in scanline bow at the photoreceptor plane produced by bending the wobble correction mirror parallel to the optical face.
In accordance with the present invention, a mirror mount adjusts the curvature of a cylindrical mirror along the optical axis of the mirror, typically the wobble correction mirror, to adjust the scanline bow for an optical scanner. By adjusting the curvature in the mirror mount of the cylindrical axis of the mirror in the plane parallel to the mirror, the scanline bows among multiple ROS""s can be approximately equalized. A mirror mount has two fixed abutments on the upper edge of the cylindrical mirror and two moveable abutment points along the lower edge of the mirror. The mirror mount adjusts the curvature of the cylindrical mirror vertically for a horizontal beam to adjust the scanline bow for a single beam and approximately equalize the scanline bow for multiple. ROS units in the printing system. During the bow adjustment the cross sectional shape of the cylindrical mirror is maintained so that the cross scan spotsize is maintained during the adjustment.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.